1. Field.
This invention relates to the formation of nylon coatings on substrates by the extrusion coating process, particularly for the production of nylon coated flexible packaging films.
2. Description of the Prior Art.
Nylon has a number of attractive properties which are desirable for use in packaging films, including toughness, high-strength, and clarity. One of the prior art techniques for the formation of packaging films having a nylon layer is the so-called high-temperature or "hot" extrusion coating process in which a nylon resin is heated to a high temperature and extruded while still in a molten condition directly onto a substrate. A process of this type is shown in United States Pat. No. 3,570,748. Also, a leading producer of nylon resins (DuPont) presents the following information in one of its technical bulletins:
"In general, the processing temperatures used in extrusion coating are higher than those in the other processes. Melt temperatures of 550.degree.-575.degree.F are normally used. The higher temperatures are required to affect adhesion to the particular substrate.
The melt contacts the substrate just prior to meeting the chill roll which solidifies or quenches the melt."
An extrusion coating process as described above has a number of distinct disadvantages. The high melt temperatures employed therein, coupled with the fact that the nylon is molten when it contacts the substrate web, causes distortion of substrates which are heat sensitive, i.e. which can become physically distorted or perhaps chemically degraded when subjected to the high temperature molten nylon. Plastic films such as polyethylene and polypropylene are in this class. Secondly, the high temperature molten nylon web when extrusion coated in such fashion will tend to remove moisture from the substrates; this is a particular problem when cellophane film is being extrusion coated because removal of moisture contained in the cellophane will cause it to become brittle. Thirdly, nylons characteristically have long draw-down distances which make it difficult to achieve accurate gauge control across the width of the web when it is extrusion coated in such process. Fourthly, the type of bonding obtained in the practice of this process, while adequate in some instances, is in general of a relatively low level. Lastly, nylon extruded at the higher temperatures typical of such processes will form a coating with a relatively high degree of crystallinity.
The above disadvantages have served to limit the production of nylon extrusion coated packaging films and thereby restrict the types of nylon extrusion coated films which are available to the flexible plastic packaging user.